The successful storage of perishable packaged foods, pharmaceuticals, and other consumable goods often depends upon the duration and temperature of storage. The useful life of such goods, their shelf life, can be reduced significantly if storage occurs at temperatures higher than those recommended. Despite precautions, a number of sources may inadvertently cause high temperatures: power outages, equipment breakdowns, inaccurate thermostats, warm sections in refrigeration units, unaccountable removal and replacement of the goods, and the like.
Such vagaries prompt the use of expiration dates, which are usually conservative estimates of shelf life and which risk premature disposal of useful goods. These uncertainties also risk use of deteriorated goods. To avoid such risks, devices that indicate whether perishable goods have accumulated a deleterious amount of thermal exposure prove useful.
Many devices and materials have been developed to indicate, by reflecting the accumulated time of storage at a predetermined temperature, expiration of shelf life. Also known in the art are cumulative dosimeters which reflect the combination of time and temperature of storage. Cholesteric liquid crystals broadly useful in the practice of the present invention comprise cholesteric compositions such as those described in U.S. Pat. No. 4,066,567, issued to Mortimer M. Labes and incorporated herein by reference.
Briefly, these compositions include a cholesteric liquid crystal compound with one or more of the constituents of the composition adapted to enter into a chemical reaction. The rate of that reaction is proportional to both the time and temperature of exposure in a cumulative manner. The light reflecting properties of the composition change in proportion to the degree of completeness of the reaction, usually causing the composition to change color. An extraneous material, such as a solute or diluent, may be added to affect the color of the liquid crystals. Thus, the compositions show a color change exhibiting, for example, that the composition has been exposed to a temperature in the range of 0.degree.-25.degree. C. for a period of several hours.
The two (or occasionally more) constituents of the compositions which will react chemically must be separately stored before use. Separate storage within the closed system of the cumulative dosimeter container is necessary to prolong the shelf life of the dosimeter. Consequently, it is also necessary to provide a quick and convenient way to mix the constituents in the closed system when the dosimeter is to be used.
A number of flexible containers have been devised, in various configurations, to permit separate storage and convenient mixing. Typically, these containers have multiple chambers or compartments separated by rupturable or frangible barriers. U.S. Pat. No. 4,469,452 issued to Sharpless et al., for example, discloses a cholesteric liquid crystal system which has a temperature sensitive membrane located between the constituents. The membrane prevents mixing until the correct temperature is reached, after which the reaction starts.
Another reference (U.S. Pat. No. 4,533,640 issued to Shafer) shows an accumulated thermal exposure device which includes an inner tube filled with one constituent reactant and an outer tube filled with a second constituent reactant. The reactants mix to form an amine and an indicator is: included. One embodiment specifically shows a reactant package which has a well of one material and a plug of another separated by a frangible barrier. Upon application of pressure, the two reactants are allowed to mix and, if the temperature is correct, to react.
The Sharpless et al. and Shafer references are typical of the prior art containers. Each shows an accumulated thermal exposure device with structural barriers physically attached to the container for separating two components. The device is activated upon removal of the barrier, either when the correct thermal conditions are attained or by application of mechanical pressure.
Containers which have a frangible barrier have several drawbacks. For many containers of this type, the user must mechanically break or rupture the barrier. That action requires great care, because the user must avoid damage to the container itself. The act of breaking the barrier without damaging the container is especially difficult when the barrier is physically attached to the container; the point at which the barrier breaks must be controlled.
In most, if not all, of the containers using a frangible barrier, the strength of the barrier and, therefore, the force needed to break it, depend, in substantial part, upon the physical characteristics of the barrier material. Thus, the physical specifications of the barrier must be controlled precisely, a requirement which increases the cost of the container. Moreover, the relatively complex structure of such frangible barriers increases the cost of manufacture.
To overcome the shortcomings of existing liquid crystal cumulative dosimeter containers, a new container is provided. An object of the present invention is to provide a container which separates the reactants until mixing is desired and which achieves separation with a barrier that is both economical to manufacture and easy to use. A related object is to avoid the need for structural barriers which are physically attached to the container. Another object is to incorporate a barrier which provides good seal integrity, preventing premature mixing, yet minimizes the chances of inadvertent rupture. It is still another object of the present invention to permit activation without damage to the container itself.